Swift: Extract type aliases.

Author: geoffw0

swift/extractor/translators/DeclTranslator.cpp

@@ -206,6 +206,7 @@ std::optional<codeql::AssociatedTypeDecl> DeclTranslator::translateAssociatedTyp
 std::optional<codeql::TypeAliasDecl> DeclTranslator::translateTypeAliasDecl(
     const swift::TypeAliasDecl& decl) {
   if (auto entry = createNamedEntry(decl)) {
+    entry->aliased_type = dispatcher.fetchLabel(decl.getUnderlyingType());
     fillTypeDecl(decl, *entry);
     return entry;
   }

swift/ql/lib/swift.dbscheme

@@ -670,7 +670,8 @@ param_decl_property_wrapper_local_wrapped_vars(  //dir=decl
 );
 
 type_alias_decls(  //dir=decl
-  unique int id: @type_alias_decl
+  unique int id: @type_alias_decl,
+  int aliased_type: @type_or_none ref
 );
 
 class_decls(  //dir=decl

swift/schema.py

@@ -324,7 +324,18 @@ class OpaqueTypeDecl(GenericTypeDecl):
     opaque_generic_params: list["GenericTypeParamType"]
 
 class TypeAliasDecl(GenericTypeDecl):
-    pass
+    """
+    A declaration of a type alias to another type. For example:
+    ```
+    typealias MyInt = Int
+    ```
+    """
+    aliased_type: Type | doc("aliased type on the right-hand side of this type alias declaration") | desc("""
+        For example the aliased type of `MyInt` in the following code is `Int`:
+        ```
+        typealias MyInt = Int
+        ```
+    """)
 
 class ClassDecl(NominalTypeDecl):
     pass